A.C. MAGNETIC STIRRING MODIFIER FOR CONTINUOUS CASTING OF METALS

Abstract: An induction stirring method wherein molten metal is electromagnetically stirred during continuous casting in a mold includes control of meniscus depression, either to decrease or enhance the stirring of the molten metal produced by the main electromagnetic stirrer. An A.C. magnetic stirring modifier is positioned adjacent the region of meniscus to produce electromagnetic stirring of the molten metal at the meniscus, either to oppose the rotary motion of the main electromagnetic stirrer and provide a surface free from turbulence or to enhance the rotary motion of the main magnetic stirrer. These two alternative modes of operation permit a casting machine to be used for casting molten metals requiring widely varying operating conditions.

Description: TITLE OF INVENTION A.C. MAGNETIC STIRRING MODIFIER FOR CONTINUOUS CASTING OF METALS FIELD OF INVENTION The present invention relates to the continuous casting of metals and alloys, for example, steel. BACKGROUND OF INVENTION In continuous steel casting by pouring liquid metal into an open-end mold, stability of the free surface of the metal in the mold, often called the meniscus, plays a significant role in both process control and the quality of as-cast product. Electromagnetic stirring of liquid steel within the mold, commonly known as M-EMS or simply EMS, is broadly employed in continuous casting mainly to improve quality of the strand surface/sub-surface and solidification structure (i.e., structure refinement, soundness and chemical homogeneity) . The two most common practices of continuous steel casting impose entirely opposite requirements to the stirring conditions within the region of molten metal near its free surface at the mold top, i.e. the meniscus region. Accordingly, casting mainly Al-killed steel grades via a submerged entry nozzle under mold powder requires meniscus stability in order to prevent disruption of mold lubrication and powder entrapment into the cast body. A rotary stirring motion at the meniscus causes meniscus depression in the centre, waves, and excessive erosion of the casting nozzle when stirring intensity exceeds a certain level. On the other hand, casting of Si-Mn deoxidized steel with an open stream is often accompanied by the defects of cast product surface. Pinholes, blowholes, surface slag entrapment and subsurface inclusions are examples of those surface defects. In order to minimize or eliminate the surface defects, an intensive stirring motion of the molten metal is required in the meniscus region. The same requirement applies for casting low deoxidized, or so-called rimming substitute steel. However, overly intensive stirring motion of the meniscus may cause undesirable deterioration of the surface by producing deep oscillation marks and lappings. Maximizing stirring intensity within the portion of metal confined by the mold is beneficial for obtaining improvements of the internal quality of cast material. Improvements in the solidification structure refinement and soundness, especially in high-carbon and certain alloy steels, strongly respond to the intensity of stirring. Having a rather limited operating space determined by the mold length, it is difficult to satisfy, with a single system, varying stirring requirements without significantly compromising some of them. For example, the limitation imposed by an acceptable level of meniscus disturbances with casting under mold powder restricts maximizing stirring intensity induced by EMS within the bulk of liquid pool. Attainment of different stirring conditions in the mold is difficult, if possible at all, when more than one of the casting techniques mentioned above are being practised at the same casting facility. There are some known methods in prior art with the objective to change or control the stirring motion in the meniscus area. Japanese Patent Publication No. 58-23554 describes a method of decreasing the intensity of stirring in the meniscus region by means of an induction coil arranged on the mold adjacent to that region and providing rotating stirring motion opposite to that induced by the main EMS coil arranged below. The main drawback of this method is that the induction coil adjacent to the meniscus region provides only a deceleration of the stirring velocity produced by EMS. In the case when an intensive stirring action within the meniscus region is required, this method would need to relinquish its decelerating action by de- energizing the coil. The coil is not intended to enhance stirring action if the need of such enhancement arises, for the reasons discussed above. In addition, the range and criterion of stirring deceleration are not disclosed in the publication. For example, a complete or near to it deceleration of stirring velocity can be related to meniscus flatness. The meniscus depression caused by rotational motion is proportional to angular stirring velocity, as seen from the expression: 2 R 2 h = (1) 2g where, h is the depth of meniscus depression W is the angular stirring velocity R is the radius of the stirred pool g is the acceleration due to gravity The depth of meniscus depression h approaches zero when the angular stirring velocity at the meniscus caused by EMS is equalized by counter-stirring angular velocity produced by a braking induction coil. Without such equalizing, the merits of that method for application to the casting under the mold powder will be limited. Another possible way of alleviating the problem of meniscus instability and decreasing stirring motion at the surface is an application of a strong horizontal D.C. magnetic field to the meniscus region. Such a field produces an electromagnetic (Lorentz) force directed opposite to the liquid metal motion and thereby reduces that motion velocity, providing a quiescent surface. An application of this concept is described in the U.S. Patent No. 4,933,005 of June 12, 1990, assigned to the assignee hereof. An electromagnetic volume force will be produced in either of two situations, firstly, when an A.C. rotating magnetic field interacts with liquid metal which is in the state of complete rest, the metal will be set into a motion with a velocity lower than that of the A.C. field; and, secondly, when a stationary, i.e. D.C. magnetic field, interacts with liquid metal already in motion. The volumetric magnetic force is proportional to velocity slip, i.e., the difference between the velocities of magnetic field and liquid metal, in accordance with the relationship: F r = 0.5 σ B 2 (W f - W m ) . R (2) In this expression describing a tangential component of the electromagnetic force produced by a two-pole induction stirrer, the parameters are: σ is the electrical conductivity of liquid metal B is the magnetic flux density W f is the angular velocity of magnetic field W m is the angular velocity of liquid metal R is the radius of liquid metal pool In the case of electromagnetic stirring with an A.C. magnetic field, W f » W m , while W f = 0 for a D.C. magnetic field and W m is comparatively low in both cases. Thus, the velocity slip between an A.C. magnetic field and moving liquid metal is much greater than that between the moving metal and a stationary field. Therefore, in order to attain magnetic force of the same value, a much greater value of magnetic flux density B is employed for a D.C. magnetic field than would be required for an A.C. magnetic field. This demand for magnetic flux increases with the requirement to reduce metal velocity to a lower level. Laboratory experiments have shown that a magnetic flux density of the order of 300 mT was required to reduce a stirring velocity in a pool of mercury by 70 to 90 percent. Considering a scaling-up effect of industrial application, it is difficult to attain both the level of magnetic field intensity and the degree of stirring velocity reduction. SUMMARY OF INVENTION In accordance with the present invention, there is provided an improved method of controlling electromagnetic stirring intensity within a continuous casting mold. This method provides both the flexibility of adaptation of stirring conditions to the casting processes and the accuracy of stirring control which were lacking by prior art. In the present invention, an electromagnetic A.C. coil similar to but smaller than that of a main electromagnetic stirrer installed downstream is arranged around the mold in the meniscus area. This device is in essence another induction stirrer, similar to the main stirrer which is arranged axially symmetrical around the mold and farther down from the meniscus. However, the coil in the upper part of the mold is intended to counterbalance and equalize, or enhance, depending on specific objectives, the stirring motion in the adjacent volume of liquid metal, the metal motion which is originated by the main stirrer. Therefore, the working function of this stirrer is to modify the pattern and/or intensity of the stirring induced by the main stirrer and henceforth the device performing that function will be called A.C. magnetic stirring modifier or A.C. MSM. The action of the A.C. MSM is typically contained within the upper portion of molten metal pool, comprising approximately 10 to 15 percent of its volume confined by the mold. The stirring motion within that portion of the metal pool is caused and maintained by the inertia forces, i.e. velocity drag, which transmit the momentum from the part of the pool where the motion has been commenced by M-EMS. Therefore, the stirring velocity at the meniscus area is less than that in the area of maximum value of M-EMS magnetic flux density, i.e. the area corresponding to the middle of EMS coil. Consequently less magnetic energy is required to counterbalance the kinetic energy of molten metal motion in the meniscus area than is provided by the M-EMS coil. Being a part of a single magnetohydrodynamic system, both the A.C. MSM and M-EMS operate at a common frequency determined by the parameters of the mold. The current supplied to both sets of the coils can be of the same variable value or it can be controlled separately. These operating features conveniently allow for a single power source for both sets of coils. The invention is broadly applicable to all electroconductive materials, i.e. metals and alloys, which can be electromagnetically stirred and where control of stirring intensity is required within some region or regions without interference with stirring within other regions of the liquid pool. The invention is applicable to a wide variety of spacial orientation of a vessel containing the molten method. For example, a casting mold may be arranged vertically, inclined or horizontally. BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a schematic of an arrangement of an A.C. magnetic stirring modifier and an electromagnetic stirrer (EMS) , with respect to a casting mold in accordance with one embodiment of the invention; Figure 2 is a schematic representation of the magnetic flux density axial profiles for the A.C. magnetic stirring modifier and the EMS of Figure 1 and the axial profile of rotational stirring velocity produced thereby; Figure 3 is a graphical representation of the relationship of meniscus depression without and with an A.C. magnetic stirring modifier at varying current of an EMS; Figure 4 is a single-line diagram of possible electrical connections for the induction coils of the A.C. magnetic stirrer modifier and the EMS of Figure 1; and Figure 5 is an elevational sectional view of the mechanical arrangement of the A.C. MSM and the EMS within the mold housing and corresponding to the schematic arrangement of Figure 1. DESCRIPTION OF PREFERRED EMBODIMENT Referring to the drawings, Figure 1 is a schematic depiction of an arrangement of an A.C. MSM and an EMS within a mold housing assembly of a continuous casting machine 10 in accordance with one embodiment of the present invention. Figure 5 is a more detailed depiction of the mechanical elements of the mold assembly. A series of induction coils 12, is arranged equally spaced around the periphery of a vertical casting mold 14, at its lower portion to comprise an A.C. electromagnetic stirrer (EMS). The EMS coils 12, when energized, induce rotary motion of a strand of molten metal 16 within the mold 14 about its longitudinal axis. A casting ceramic tube 18 is axially located with respect to the strand of molten metal 16, when casting with a submerged entrance nozzle is being performed. In accordance with the present invention, A.C. MSM induction coils 20, are equally spaced around the vertical mold 14, adjacent to a free upper surface or meniscus 22 of the strand of molten metal 16. The EMS coils 12 are designed to induce a strong rotational flow of molten metal in the strand of molten metal 16 within the mold 14. The intensity of this rotational flow is characterized by its rotational velocity U R which, in turn, depends on the parameters comprising the expression: , U h R K ϊ ' L (3) wherein, K is a proportionality coefficient T is the magnetic torque applied to the molten metal L is a characteristic length of stirrer The magnetic torque T is defined by other parameters of the electromagnetic system: T = 0.5 π 2 ■ f ■ a ■ B 2 • R 4 (4) where, f is the current frequency σ is the liquid metal electrical conductivity B is the magnetic flux density R is the stirred pool radius A maximum value of rotational velocity is attained within and about the region of molten metal defined by a characteristic length of stirrer L which corresponds to a magnetic flux density B distribution along stirring axis. A typical magnetic flux density distribution for the two sets of induction coils 12 and 20 are shown in Figure 2. The axial distribution of rotational stirring velocity, U R , also is presented in Figure 2. As seen from this schematic, the rotational velocity U R extends well beyond the active stirring zone L of the EMS coils 12 within the molten metal pool 16. This effect is due to the fact that angular momentum originated by the stirring coils 12 is transported by secondary poloidal flow within the metal strand 16. The secondary flow is originated at the stirring symmetry area, i.e., the stirrer neutral horizontal axis, and is directed from the active stirring zone L along the solidification front, making a return loop towards the stirrer at the stirred pool centre. The value of the maximum stirring velocity within and about the active stirring zone L and the rate of its axial attenuation within the metal 16 determine the stirring velocity at the meniscus area 22 in the absence of other effects. Along with the value of magnetic flux density and the frequency, the stirring velocity value and its lengthwise axial range depend on the stirrer length L, the radius of the stirred pool R, and the roughness of the solidification interface with liquid metal. Accordingly, it is difficult to quantitatively and accurately predict the stirring velocity at the meniscus, based upon the design and operating parameters of the EMS coils 12 and the distance from EMS neutral axis to the meniscus. For a typical EMS arrangement within a steel billet/bloom continuous caster mold housing, the stirring velocity at the meniscus generally is about 0.5 to 0.7 (about 50 to 70 percent) of maximum stirring velocity value while the EMS coils 12 are located at a lowest position with respect to the meniscus. Therefore, a substantial stirring action can be expected at the meniscus area produced by the EMS coils even if the latter is located at the farthest possible distance from the meniscus. Meniscus depression and, more generally, turbulence at this location manifest themselves as a result of this stirring action. The meniscus depression depth, as was shown earlier in the expression (1) , is strongly correlated to the angular stirring velocity at the meniscus. At given EMS design parameters, for example, active stirring length, power input, frequency and distance from the meniscus, the meniscus stirring velocity and depression are proportional to the current supplied to the EMS coils 12, as shown schematically in Figure 3. Depending on the factors mentioned above, the meniscus depression for industrial systems can range from approximately 6 to 27 mm, for example. In order to counterbalance the stirring velocity at the meniscus area produced by the EMS coils 12, in accordance with the present invention, the induction coils 20 of A.C. MSM are energized, to induce a stirring action within the liquid metal at the meniscus opposite to that caused by the EMS coils 12. All the previous considerations with respect to a rotary movement of liquid metal are applicable to the stirring produced by the A.C. MSM coils 20. The A.C. MSM coils 20 are substantially smaller and require less power for their operation than the EMS coils 12 due to a much less stirring velocity expected for them to produce to counteract the rotational motion at the meniscus induced by the EMS coils 12. In accordance with an embodiment of this invention, the A.C. MSM coils 20 are energized from a power supply common with the EMS coils 12, as shown by single line diagrams in Figure 4. Schemes I and II appearing in Figure 4 show the A.C. MSM and EMS coils 20 and 12 respectively connected in series and, therefore, operating at the time same current and frequency supplied from a common power source. The coil connections presented in Scheme I provide for unidirectional rotating magnetic fields produced by both the EMS and A.C. MSM coils. This mode of operation is employed for enhancing the stirring motion at the meniscus area by the A.C. MSM coils 20. The coil connections presented in Scheme II provide for counter-rotating magnetic fields and cause counter-rotating liquid metal motions in the areas corresponding to the EMS and A.C. MSM coils. In order to provide fine control over stirring action at the meniscus which is determined by the variables of EMS (for example, B-flux) , the current level supplied to the A.C. MSM coils 20 may have an independent control from that of the EMS coils 12, as shown by Scheme III in Figure 4. This arrangement allows for independent control of stirring actions of either of the EMS or the A.C. MSM coils regardless of the directional pattern of stirring, namely unirotational or counter-rotational. The independent control of stirring motion at the meniscus provided by the use of the A.C. MSM coils 20 enables a greater flexibility of the stirring process control with a possibility of achieving equalization of the opposite stirring motions at the meniscus, and minimization of its depression, as illustrated in Figure 3. As seen therein, the line OA corresponds to the meniscus depression caused by the stirring induced by EMS coils 12 without being opposed or added by A.C. MSM stirring. Similarly, the line OD represents meniscus depression associated with isolated stirring action induced by the A.C. MSM coils 20. In order to equalize the stirring velocities caused by the EMS and A.C. MSM coils, the meniscus depression must be of the same value in either of the situations. For example, if the meniscus depression caused by EMS stirring corresponds to the level A, then counter-rotational stirring provided by A.C. MSM stirring should have corresponding meniscus depression, i.e. level D. The line OC is the resultant of two opposite stirring actions produced respectively by the EMS and AC MSM coils and equalized at the meniscus. The line AB represents the resultant of two unidirectional stirring actions. In this case, the range of stirring enhancement expressed through the meniscus depression can be adjusted in accordance with the casting practice requirements, so that the stirring intensity of EMS is fully utilized. SUMMARY OF DISCLOSURE In summery of this disclosure, the present invention provides an improved method of controlling disturbance of the free surface of molten steel or other metal or alloy being cast through a mold and caused by electromagnetic stirring applied to the liquid metal, to minimize such disturbance or achieve an enhanced, within a single casting unit stirring motion at the meniscus, by employing an induction modifier in the form of an electromagnetic stirrer adjacent to the location of the meniscus. Modifications are possible within the scope of this invention.

Claims: CLAIMS What we claim is: 1. An induction stirring method for liquid metals, which comprises: electromagnetically inducing stirring of molten metal with such intensity as normally to result in turbulence in the molten metal including its free surface, by applying a first rotating magnetic field to said molten metal, and applying a second rotating magnetic field produced by a source separate from that providing said first magnetic field and at a location upstream of said stirring of an intensity which selectively is (a) at least sufficient to minimize the stirring motion and disturbances in said free surface area when the second rotating magnetic field is directed opposite to the direction of the first rotating magnetic field, or (b) to enhance stirring motion in said free surface area when said first and second rotating magnetic fields have the same direction of rotation. 2. The method of claim 1, wherein said second rotating magnetic field is applied in a location adjacent the free surface area of said molten metal. 3. The method of claim 2, wherein the second rotating magnetic field is provided by a set of induction coils controlled by an A.C. current supplied from a power source common to and shared with a set of induction coils producing said first rotating magnetic field. 4. The method of claim 2, wherein the second rotating magnetic field is provided by a set of induction coils controlled by an A.C. current to an independent power source from a power source for a set of induction coils producing said first rotating magnetic field. 5. The method of claim 3 or 4, wherein the sets of induction coils are coils of multi-phase and multi-pole arrangement spaced peripherally around the area of the molten metal in their location of provision of the respective rotating magnetic field. 6. The method of claim 1, wherein the second rotating magnetic field employed to effect a stirring motion in the meniscus area sufficient to suppress stirring motion of intensity of about 50 to 70 percent produced by the first rotating magnetic field at its downstream location of application. 7. The method of claim l, wherein the second rotating magnetic field is employed to effect a stirring motion in the meniscus area sufficient to enhance that stirring motion to a level, at or near the original stirring intensity produced by the first rotating magnetic field at its downstream location of application. 8. The method of claim 1, wherein said liquid metal is confined in a vertically-oriented continuous casting mold in which the first rotating magnetic field is applied at a downstream location and said second rotating magnetic field is applied in the free surface area of the liquid metal vertically-spaced from said downstream location. 9. The method of claim 8 wherein said liquid metal is steel.

Inventors: BEITELMAN LEONID, MULCAHY JOSEPH A, BEITELMAN, LEONID, MULCAHY, JOSEPH, A.
Applicants: MULCAHY J ENTERPRISES INC, J. MULCAHY ENTERPRISES INC.
Application year: 1994
Main classification: B22D11
IPC codes: B22D11